Petrogenesis of the gabbros from Mt. St. Hilaire,Quebec, Canada

Mt. St. Hilaire occurs as a small funnel-shaped intrusion in the Monteregian petrographic province of Quebec and consists of alkali gabbros and later nepheline syenites. Based on field relations, petrography, and geochemistry, five types of gabbro are recognized. In order of intrusion these are: leucogabbro, foliated gabbro, kaersutite-biotite gabbro, kaersutite gabbro, and a gabbro-melagabbro series. Based on analyses of the early-forming ilmenite-titanomagnetite, the gabbros crystallized under high fO₂ conditions which lead to subsequent crystallization of olivines with high MnO contents. Fractionation of ilmenite and titanomagnetite was a major control on the Ti and A[TV]concentrations in the clinopyroxenes. Plagioclase compositions in the gabbros became richer in Ab contents in the sequence gabbro-melagabbro to leucogabbro. Whole-rock analyses suggest that the parental magma of alkali basaltic composition was fairly evolved prior to emplacement. Lack of olivine in the cumulate gabbro-melagabbros and low Ni and Cr in all gabbros may reflect either extreme olivine fractionation and/or a very low olivine content in the source material for these basalts. Differentiation of the gabbros occurred both pre- and post-emplacement, probably by a process of crystal-liquid fractionation at depths between 3-5 and 8 km. This is in accordance with geophysical measurements for other Monteregian intrusions. A model is presented for the mechanism of emplacement.